RESUMEN
Oxidative stress is believed to play a role in the pathogenesis of several diseases, including diabetes and inborn errors of metabolism. The types of oxidative damage observed in these pathologies have been attributed to the excessive production of reactive intermediates relating to the accumulation of toxic metabolites. The production of extremely oxidizing peroxynitrite can also be high in these pathologies. We study here the oxidation initiated by peroxynitrite of the ethyl esters of acetoacetate (EAA) and 2-methylacetoacetate (EMAA), metabolites that accumulate in diabetes and isoleucinemia, respectively. Oxygen consumption studies have confirmed that peroxynitrite promotes the aerobic oxidation of EAA and EMAA in phosphate buffer. These reactions were accompanied by ultraweak light emission, which probably arises from triplet carbonyl products formed by thermolysis of dioxetane intermediates. The kinetics of oxygen uptake and chemiluminescence by EAA and EMAA was strongly affected by the phosphate ion, known to catalyze carbonyl enolization and nucleophilic additions to carbonyls. The reaction pH profiles obtained by oxygen consumption and chemiluminescence measurements indicated that the peroxynitrite anion was the initiator of EAA and EMAA aerobic oxidation. EPR spin-trapping studies with the spin traps 3,5-dibromo-4-nitrosobenzenesulfonic acid and 2-methyl-2-nitrosopropane showed the intermediacy of methyl and a carbon-centered radical (*CH2COR) in the oxidation of EAA by peroxynitrite. In the case of EMAA, a tertiary carbon-centered radical (*EMAA) and an acyl radical were detected, the latter probably resulting from the cleavage of a triplet carbonyl product. Superstoichiometric formation of acetate from both substrates confirmed the occurrence of oxygen-dependent chain reactions, here proposed to be initiated by one-electron abstraction from the enolic form of the substrates. The free radicals and electronically excited species generated in the oxidation of EAA and EMAA may help shed further light on the molecular basis of these diseases.
Asunto(s)
Acetoacetatos/metabolismo , Estrés Oxidativo , Oxígeno/metabolismo , Ácido Peroxinitroso/química , Cetoacidosis Diabética/etiología , Espectroscopía de Resonancia por Spin del Electrón , Radicales Libres/metabolismo , Concentración de Iones de Hidrógeno , Mediciones Luminiscentes , Oxidación-Reducción , Ácido Peroxinitroso/toxicidadRESUMEN
Hereditary tyrosinemia type I (HT1) is an inborn metabolic error characterized by hepatorenal dysfunction. Affected patients excrete large quantities of succinylacetone (SA), a tyrosine catabolite believed to be involved in the pathogenesis of HT1. A growing body of evidence relates the oxidative stress observed in metabolic disorders to free radicals generated from accumulated metabolites. In this context, oxidation of SA by peroxynitrite or cytochrome c yielding reactive intermediates and products was investigated here. Both peroxynitrite and cytochrome c were able to initiate oxygen consumption by SA, which was followed by polarimetric and chemiluminescence measurements. The light emission arises from triplet carbonyls formed by the thermolysis of dioxetane intermediates, as indicated by energy transfer experiments. EPR spin-trapping studies with 2-methyl-2-nitrosopropane revealed the intermediacy of two different carbon-centered radicals, one of them originating from cleavage of the triplet carbonyl product. The pH profiles obtained by oxygen consumption, chemiluminescence, and stopped-flow spectrophotometry point to the peroxynitrite anion as the initiator of SA aerobic oxidation. Overstoichiometric formation of organic acids based on added peroxynitrite confirms the occurrence of an oxygen-dependent chain reaction, here proposed to be initiated by one electron abstraction from the enolic form of SA. The results obtained may help shed light on the role of both SA and oxidative stress in the pathogenesis of HT1.